I have a mailing list application that sends emails through several dedicated SMTP servers (running Linux Debian 5 and Postfix) in the same network of a hosting company.

However, the application is using the servers' external IP addresses in order to connect to them over SMTP, and I was wondering what kind of improvement would be obtained if the application used the internal IP addresses of the servers instead?

Depending on the network compontents "between" the application and the (SMTP-)server (like firewalls, routers, switches...) it can differ a lot, but in most cases using the internal address should be faster.

This question is more a network question and should be asked to the people responsible for this specific network.